There have been numerous reports on the quenching of chlorophylla(Chla) fluorescence by various oxidizing and reducing agents.1–10On the basis of the dependence of the quenching constants on the redox potentials of the quenchers, an electron transfer mechanism has been proposed although there is no experimental evidence for the radical ion pair formation. A survey of literature on chlorophyll fluorescence quenching shows that there have been no studies of singlet quenching of chlorophyllb(Chlb) so far. On the basis of singlet excited state halfwave potential values, Chlbwould be expected to